Tears are formed by tiny glands that surround the eye. The tear film is comprised generally of three layers including oil, water, and mucous. The lower mucous layer generally serves to anchor the tear film to the eye. The middle layer is comprised of water while the upper oil layer seals the tear film and prevents or mitigates evaporation. Dry eye syndrome (DES), also known as Keratoconjunctivitis-sicca, is a disorder of the tear film due to tear deficiency or excessive tear evaporation which causes damage to the interpalpebral ocular surface and is associated with symptoms of ocular discomfort. Tear film instability may cause alteration in volume, composition, or distribution, of the tear film, thus assessing the dynamics of tear film is very important in addressing the DES.
The human tear film is the outermost layer in the eye. As explained above, it is composed of an outer oily or lipid layer over a mixture of aqueous and mucus layers. The tear film lubricates the cornea of the eye and keeps the front of the eyeball moist and clean. The tear film is also the first refractive medium of the eye and acts as shield to protect the eye from foreign objects and bacteria. Therefore, tear film is an important factor in evaluating the quality of vision, both optically and physiologically.
Analyzing the tear film quality and dynamics are complex tasks. The most commonly used diagnostic methods, such as the Schirmer test, Rose Bengal, and fluorescein staining, are invasive and can be uncomfortable for the patient. These techniques are subjective and do not provide quantitative information. They also exhibit high variability. Amongst the non-invasive techniques, high-speed videotopography has been applied to measure the tear film build-up time. Videotopography is described, for example, in J. Nemeth et al., High-Speed Videotopographic Measurement of Tear Film Build-up Time, Invest. Opthalmol. Vis. Sci. 43(6), 1783-1790 (2002).
Another technology, wavefront aberrometry has been used to quantify the tear break-up time (TBUT). This technique is described, for instance, in R. Montes-Mico et al., Dynamic Changes in the Tear Film in Dry Eyes, Invest. Opthalmol. Vis. Sci. 46(5), 1615-1619 (2005). Interferometry has also been applied in measuring the thickness of the tear film. There are two different interferometry techniques that have been proposed and applied in the opthalmology research community. King-Smith et al. were amongst the pioneers in applying interferometry to tear film studies. Their approach was to analyze the reflectance spectra from the tear film using visible and near-infrared light source. King-Smith et al. analyzed the modulation and phase of the oscillations in the spectra. King-Smith et al. were able to quantify the thickness of the pre-corneal tear film and for the first time obtain a value of 3 μm. The King-Smith et al. techniques are discussed, for example, in King-Smith et al., The thickness of the tear film, Current Eye Research, pp. 357-368, Informa Healthcare, London (2004); King-Smith et al., The Thickness of the Human Precorneal Tear Film: Evidence from Reflection Spectra, Invest. Opthalmol. Vis. Sci. 41(11), 3348-3359 (2000); King-Smith et al., Thickness of the Pre- and Post-Contact Lens Tear Film Measured In Vivo by Interferometry, Invest. Opthalmol. Vis. Sci. 44(1), 68-77 (2003); and King-Smith et al., Three Interferometric Methods for Measuring the Thickness of Layers of the Tear Film, Optometry & Vision Science 76(1), 19-32, (1999).
Licznerski et al. modified the lateral shearing interferometry method and initiated a cascade of applications by scientists in the field. The Licznerski et al. is described, for example, in T. J. Licznerski et al., Analysis of Shearing lnterferograms of Tear Film Using Fast Fourier Transforms, Journal of Biomedical Optics 3(1), 32-37 (1998). This method was mainly applied by Szczesna et al. to evaluate the dynamics of the tear film. Szczesna et al. were able to assess the stability of the tear film on the cornea and on contact lenses. In this method, interference patterns undergo a fast Fourier transformation to analyze the changes in the orientation of the fringes in interferograms. Quantitative information can be revealed from the spectra in the fast Fourier transformed images as described in Szczesna et al., Numerical analysis of interferograms for evaluation of tear film build-up time, Ophthalmic and Physiological Optics 29(3), 211-218 (2009) and Szczesna et al., An interferometric method for the dynamic evaluation of the tear film, Acta Opthalmologica Scandinavica 85(2), 202-208 (2007). However, these techniques have not been applied routinely yet.
Optical coherence tomography (OCT) has also been applied to tear film measurements. OCT is a non-invasive cross-sectional imaging methodology in biomedical applications. Low coherence light source is applied in the OCT systems to obtain a two dimensional image that could reveal optical characteristics of the specimen. However, OCT generally does not have the resolution in the axial direction to directly measure the tear film. Wang et al. has, however, used a commercial OCT to indirectly measure the thickness of the normal pre-corneal tear film. This is described in Wang et al., Precorneal and Pre- and Post lens Tear Film Thickness Measured Indirectly with Optical Coherence Tomography, Invest. Opthalmol. Vis. Sci. 44(6), 2524-2528 (2003).